Centralize handling of in-test dumpering
Remove many of the settled-by-time comments, modernize a bit
Normalize handling of expected warnings/exceptions in tests
Move test to a better location (no changes)
Extensive tests and deprecations of multivalue/no value inequality op calls

All sql-generation tests match pre <= 1.74 behavior (except for the newly
added rlike operator, which now warns while it simply did not work with
undefs before)
Reintroduce handling of IS( NOT) ops

This is undocumented syntax, but it is very likely someone is using it in
the wild, and the previous inequality op handler rewrite broke this
Port fce4b7a9 from the DQ branch (with warning expectation ammendments)